This invention relates generally to magnetic resonance imaging (MRI) and, more particularly, the invention relates to echo planar imaging using partial flyback.
Magnetic resonance imaging (MRI) is a non-destructive method for the analysis of materials and represents a new approach to medical imaging. It is generally non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radio-frequency signals resulting from the precession of these spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
Echo planar imaging (EPI) uses a gradient scanning of k-space in which data is acquired in both positive and negative directions along one axis (k.sub.x) while the scanning is moved incrementally along another axis (k.sub.y). As described by Butts and Riederer, "Analysis of Flow Aspects in Echo-Planar Imaging," Journal of Magnetic Resonance Imaging, June 1992, the reversing of scan directions causes blurring, ghosting, and signal loss when material flows in the imaged plane. Artifacts due to flow in the phase-encoded direction can be reduced by using a partial k-space trajectory, as disclosed by McKinnon, "Ultrafast Interleaved Gradient-Echo-Planar Imaging," Magnetic Resonance in Medicine 30:609-616, 1993. Flow artifacts are reduced because there is less phase-encoding before the center of k-space is acquired. The flyback partial Fourier trajectory reduces readout-flow artifacts by acquiring data in only one direction along the readout axis (k.sub.x). See Duerk and Simonetti, "Theoretical Aspects of Motion Sensitivity and Compensation in Echo Planar Imaging," JMRI 1991, 1:643-650. However, this approximately doubles the time to acquire an image.
The present invention is directed to a partial-flyback EPI where the flyback trajectory is used at the center of k-space and an echo planar trajectory is used elsewhere in acquiring data.